This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. DNA synthesis past deamination products of cytosine and 5-methylcytosine-containing cis-syn pyrimidine dimer photoproducts may be one of the principal mechanisms by which sunlight induces C-to-T transition mutations linked to skin cancer. To understand better the structure-activity relationships governing photoproduct deamination, we designed the ODN sequence d(GTATCATGAGGTGC) containing a unique TC site to enable preparation of a dT[c,s]C dimer by UV irradiation to measure the kinetics of deamination of cytosine photoproducts. Instead of the expected T[c,s]C photoproduct, an abundant unknown with an unusually short HPLC retention time attracted our attention during the course of UVB irradiation (312 nm) of d(GTATCATGAGGTGC). This product only appeared at low pH (below 6). Our goal is to elucidate the structure of this unknown major photoproduct by combining various techniques such as HPLC, mass spectrometry, NMR, and X-Ray crystallography.